Piping systems used in the water works industry are subject to axial thrust forces that are developed whenever there is a change in the direction of flow. It is necessary to have a means of resisting the thrust forces, or the piping system may separate and leak at the joined connections. Concrete “thrust blocks” were the primary method of resisting thrust forces for much of the twentieth century. In the 1960s, the concept of a mechanical device to provide thrust restraint was developed. There are currently several different mechanical methods and devices for providing thrust restraint. Some of these devices provide thrust restraint and may also be used to compress a gasket to seal the piping system. Most of these devices have a circular array of gripping components, often referred to as wedges or segments, that work together with a ring, or gland, to provide uniform thrust restraint. The gland joins the gripping components and also compresses a gasket.
Another class of device provides only thrust restraint. One of the earliest devices developed only for thrust restraint and intended for use on polyvinyl chloride (PVC) pipe is a metallic ring that has serrations (a thread profile) on the surface that contacts the pipe outside diameter (OD). The ring is typically split into two (or more) equal sections. The ring is then assembled on the outside surface of the pipe and clamped together to assure the serrations penetrate into the outer surface of the pipe. The penetration of the serrations into the outer surface of the pipe prevents movement of the ring. The ring in combination with another similar restraint, or a non-serrated ring that rests on a pipe bell, connected across a joint by rods can be used to prevent joint separation.
There are two prevalent series of PVC pipe used in the water works industry. The first series has ODs that are cast-iron-pipe-equivalent (CI) and the second series has ODs that match steel-pipe-equivalent (IPS) dimensions. The ODs on 4-12″ nominal PVC distribution pipe differ by 0.27″ (6″ nominal) to 0.45″ (12″ nominal). Typically a split ring type restraint must be designed to fit one pipe series as the inner diameter (ID) of the ring is sized to allow a certain amount of interference with the OD of the pipe. It is also typical (especially on CI sized pipe) for the split rings to mate up fully on installation, often referred to as “pad-to-pad”, as an in indicator that the ring is properly installed.